Photographic film: How it works

Photographic film is essentially a thin plastic base coated with an emulsion. The emulsion is composed of gelatin within which tiny particles of light-sensitive salts have been suspended. The salts used are usually silver HALIDES, such as silver iodide. The stored reaction to light (after the film has been exposed in the camera) is called the latent image and can be seen only after the film has been processed, or developed (see FILM PROCESSING). A glass base may be coated with an emulsion for a special scientific or research purpose.

An antihalation backing is applied to the underside of the base to prevent a halo effect or flare around bright objects when light bounces back and passes through the film a second time. Overcoats are applied to the film to minimize abrasion when the film moves through the camera and when it is processed. Because of the nature of the silver halides in the emulsion, all light-sensitive photographic emulsions are sensitive to the violet and blue end of the visible spectrum. By adding different types of sensitizing dyes to the emulsion, film can be made to react to the entire spectrum or to limited portions of it. This response to different wavelengths of light is called color sensitivity. Film that is sensitive to light over the entire visible spectrum is called panchromatic and is the type used for most general photography.

By varying the amount and type of sensitizing dye used, the film's sensitivity to light intensity, which is called emulsion speed, may be adjusted. The speed figures were devised to indicate the minimum exposure necessary to produce a specified degree of blackening of the film. The American Standards Association (ASA) is one organization that has developed a system for measuring this speed. In this system, known as the ASA Exposure Index or simply ASA, the higher the numerical rating, the faster the film's speed. Thus, a film rated at ASA 100 is twice as fast as and will require only half the exposure of a film rated at ASA 50. For the most part, the faster a film is, the more grainy it appears.

In a film negative, those areas containing the lightest objects (highlights) will have the greatest amount of density after processing, while the darkest objects (shadows) will have relatively less. Contrast is the intermediate range of tones between the very light and very dark elements in the original photograph. A high-contrast film will record only the highlights and shadows, or just white and black, whereas a medium- or low-contrast film will also record the intermediate tones. Black-and-white negative film generally has only one emulsion layer, which is sensitive more or less equally to all the colors in the visible spectrum.

After processing, the highlights appear as solid black on the negative, the shadows as clear, and the middle tones as gray. White light can be created by superimposing, or adding together, equal amounts of red, green and blue light, the additive primary COLORS. Any two of these primary colors, when combined, form yet another color, the complement, or opposite, of the third primary. For example, green and blue produce the color cyan, which is the opposite of red; red and blue produce magenta, the opposite of green; and green and red produce yellow, the opposite of blue.

The colors cyan, magenta, and blue are referred to as the subtractive primaries because each represents the remaining color after one primary color has been subtracted from white light. Color negative film emulsion is composed of three dye layers, each one sensitive to a separate component of light--red, green, or blue. During processing, these three layers form, respectively, the dye colors cyan, blue, and magenta. Thus, red will appear as cyan, green as magenta, and blue as yellow on the negative.